Journal of Physical Chemistry B, Vol.109, No.30, 14365-14374, 2005
Control of photoinduced energy- and electron-transfer steps in zinc porphyrin-oligothiophene-fullerene linked triads with solvent polarity
The dramatic changes of the lifetimes of the charge-separated (CS) states were confirmed in zinc porphyrin (ZnP)-oligothiophene (nT)-fullerene (C-60) linked triads (ZnP-nT-C-60) with the solvent polarity. After the selective excitation of the ZnP moiety of ZnP-nT-C60, an energy transfer took place from the (ZnP)-Zn-1* moiety to the C-60 moiety, generating ZnP-nT-C-1(60)*. In polar solvents, the CS process also took place directly via the (ZnP)-Zn-1* moiety, generating ZnP.+-nT-C-60(.-), as well as the energy transfer to the C60 moiety. After this energy transfer, an indirect CS process took place from the C-1(60)* moiety. In the less polar solvent anisole, the radical cation (hole) of ZnP.+-nT-C-60(.-) shifted to the nT moiety; thus, the nT moiety behaves as a cation trapper, and the rates of the hole shift were evaluated to be in the order of 10(8) s(-1); then, the final CS states ZnP-nT(.+)-C-60(.-) were lasting for 6-7 mu s. In the medium polar solvent o-dichlorobenzene (o-DCB), ZnP-nT(.+)-C-60(.-) and ZnP.+-nT-C60(.-) were present as an equilibrium, because both states have almost the same thermodynamic stability. This equilibrium resulted in quite long lifetimes of the CS states (450-910 mu s) in o-DCB. In the more polar benzonitrile, the generation of ZnP-nT(.+)-C-60(.-) was confirmed with apparent short lifetimes (0.6-0.8 mu s), which can be explained by the fast hole shift to more stable ZnP.+-nT-C-60(.-) followed by the faster charge recombination. It was revealed that the relation between the energy levels of two CS states, which strongly depend on the solvent polarity, causes dramatic changes of the lifetimes of the CS states in ZnP-nT-C60; that is, the most appropriate solvents for the long-lived CS state are intermediately polar solvents such as o-DCB. Compared with our previous data for H2P-nT-C-60, in which H2P is free-base porphyrin, the lifetimes of the CS states of ZnP-nT-C-60 are similar to 30 times longer than those in o-DCB.